The possibility to finely control nanostructured cubic ferrites (M^IIFe₂O₄) paves the way to design materials with the desired magnetic properties for specific applications. However, the strict and complex interrelation among the chemical composition, size, polydispersity, shape and surface coating renders their correlation with the magnetic properties not trivial to predict. In this context, this work aims to discuss the magnetic properties and the heating abilities of Zn-substituted cobalt ferrite nanoparticles with different zinc contents (Zn_xCo_1−xFe₂O₄ with 0 < x < 0.6), specifically prepared with similar particle sizes (∼7 nm) and size distributions having the crystallite size (∼6 nm) and capping agent amount of 15%. All samples have high saturation magnetisation (M_s) values at 5 K (>100 emu g⁻¹). The increase in the zinc content up to x = 0.46 in the structure has resulted in an increase of the saturation magnetisation (M_s) at 5 K. High M_s values have also been revealed at room temperature (∼90 emu g⁻¹) for both CoFe₂O₄ and Zn_0.30Co_0.70Fe₂O₄ samples and their heating ability has been tested. Despite a similar saturation magnetisation, the specific absorption rate value for the cobalt ferrite is three times higher than the Zn-substituted one. DC magnetometry results were not sufficient to justify these data, the experimental conditions of SAR and static measurements being quite different. The synergic combination of DC with AC magnetometry and ⁵⁷Fe Mössbauer spectroscopy represents a powerful tool to get new insights into the design of suitable heat mediators for magnetic fluid hyperthermia.

XPS is exploited to study the composition, the chemical state and the cation distribution of metals in CoFe ₂ O ₄ .

This paper aims to analyze the competition of single particle anisotropy and interparticle interactions in nanoparticle ensembles using a random anisotropy model. The model is first applied to ideal systems of non-interacting and strongly dipolar interacting ensembles of maghemite nanoparticles. The investigation is then extended to more complex systems of pure cobalt ferrite CoFe₂O₄ (CFO) and mixed cobalt–nickel ferrite (Co,Ni)Fe₂O₄ (CNFO) nanoparticles. Both samples were synthetized by the polyol process and exhibit the same particle size (D_TEM ≈ 5 nm), but with different interparticle interaction strengths and single particle anisotropy. The implementation of the random anisotropy model allows investigation of the influence of single particle anisotropy and interparticle interactions, and sheds light on their complex interplay as well as on their individual contribution. This analysis is of fundamental importance in order to understand the physics of these systems and to develop technological applications based on concentrated magnetic nanoparticles, where single and collective behaviors coexist.

The effect of the annealing temperature T_ann on the magnetic properties of cobalt ferrite nanoparticles embedded in an amorphous silica matrix (CoFe₂O₄/SiO₂), synthesized by a sol-gel auto-combustion method, was investigated by magnetization and AC susceptibility measurements. For samples with 15% w/w nanoparticle concentration, the particle size increases from ~2.5 to ~7 nm, increasing T_ann from 700 to 900 °C. The effective magnetic anisotropy constant (K_eff) increases with decreasing T_ann, due to the increase in the surface contribution. For a 5% w/w sample annealed at 900 °C, K_eff is much larger (1.7 × 10⁶ J/m³) than that of the 15% w/w sample (7.5 × 10⁵ J/m³) annealed at 700 °C and showing comparable particle size. This indicates that the effect of the annealing temperature on the anisotropy is not only the control of the particle size but also on the core structure (i.e., cation distribution between the two spinel sublattices and degree of spin canting), strongly affecting the magnetocrystalline anisotropy. The results provide evidence that the magnetic anisotropy comes from a complex balance between core and surface contributions that can be controlled by thermal treatments.